A. Field of the Invention
The present invention relates to a method of making a liquid crystal cell, and more particularly to a method of controlling pretilt of the liquid crystal cell.
B. Description of the Prior Art
To manufacture liquid crystal display device having uniform brightness and a high contrast ratio, it is necessary to align the liquid crystal molecules within the liquid crystal layer in a certain direction. The most useful aligning method is the rubbing method. In the rubbing method, the alignment layer, a polyimide or polyamide-coated layer, is mechanically brushed with fabrics, etc., so that microgrooves are generated on the surface of the alignment layer.
The periodic topology of mechanically grooved LCD-substrates minimizes the elastic deformation energy of liquid crystals by forcing the director to align parallel to the microgrooves. In the rubbing method, however, the defect of the microgrooves causes random phase distortion and light scattering, deteriorating the image quality. Further, the brushing using rubbing cloth in the rubbing process generates dust and electrostatic discharge on the alignment layer causing damage to the substrate and resultant yield deterioration.
To solve the above substrate damage and yield deterioration problem, a photo-aligning method using ultraviolet light (UV light) has been introduced (T. Hashimoto et al. Digest SID 95 p.877-880). By Hashimoto method, the alignment layer, a polyvinylcinnamate (PVCN) based polymer, is exposed to UV light in the vertical and oblique directions with respect to the surface of the alignment layer to form the pretilt direction to the alignment layer. That is, as shown in FIG. 1A, only the y-axis chains of the polymer are dimerized and the other side chains in the x-z plane remain when the alignment layer 10 is vertically exposed with the UV light having a polarization direction parallel to the y-axis.
In FIGS. 1A and 1B, the dotted arrows indicate the direction of the polymer dimerizations and solid arrows indicate the direction of the remaining side chain in the x-z plane. By vertical exposure to UV light, the optical constants in the y-z plane are directed parallel to the z-axis while the optical constants in the x-z plane still exhibit anisotropy yet. When the alignment layer 10 is obliquely exposed with ultraviolet light having a polarization direction parallel to the x-z plane, as shown in FIG. 1C, the side chains parallel to the polarization direction are dimerized, so that only the side chains parallel to the irradiating direction remain.
The remaining side chains interact with the liquid crystal molecules to give the pretilt direction to the liquid crystal molecules. At that time, the pretilt angle that will determine the tilt angle between the surface of the alignment layer 10 and the liquid crystal molecules depends upon the irradiation angle of the UV light relative to the surface of the alignment layer. When the irradiation angle is 30.degree., 45.degree., 60.degree., for example, the pretilt angle is about 0.15.degree., 0.26.degree., 0.30.degree..
However, in Hashimoto method, since the alignment layer 10 is exposed with UV light twice, the process is complicated. Further, because the generated pretilt angle is very small, a desired pretilt angle having a large size cannot be obtained.
As used in this description and in the appended claims, the word `pretilt direction` means both the alignment direction, which is the azimuthal direction of the LC molecules to be aligned on the surface of the alignment layer, and the pretilt angle between the liquid crystal molecules to be tilted and the surface of the alignment layer.
In this invention, polysiloxane based material or polyvinylfluorocinnamate (PVCN-F) is preferably used as an alignment layer. The structural formulas of the polysiloxanecinnamate and PVCN-F are indicated below. In these formulas, polysiloxanecinnamate I and II are examples of polysiloxane based materials. PVCN-F: ##STR1## Z=OH, CH.sub.3, or a mixture of OH and CH.sub.3
m=10-100 PA1 l=1-11 PA1 L=0 or 1 PA1 K=0 or 1 PA1 X, X.sub.1, X.sub.2, Y=H, F, Cl, CN, CF.sub.3, C.sub.n H.sub.2n+1 or OC.sub.n H.sub.2n+1 (n=1-10) PA1 As shown in FIG. 2, the pretilt angle of the polysiloxane or PVCN-F alignment layer can be controlled in 0.degree.-90.degree.. However, since the gradient of the characteristic curve of the absorption energy, i.e., the irradiating time of the UV light and the pretilt angle in the region .DELTA.x is too steep, the size of the pretilt angle is abruptly varied according to the irradiating time in the region .DELTA.x. Accordingly, it is very difficult to precisely control the pretilt angle in this region. Practically, the pretilt angle (3.degree.-5.degree.) in the region except for the region .DELTA.x can be controlled.
As shown in FIG. 2, the pretilt angle of the polysiloxane or PVCN-F alignment layer depends on the irradiation time or the density of the UV light, i.e., the amount of absorption energy of the UV light. The larger the absorption energy is, the smaller the pretilt angle. Accordingly, the direction of the alignment axis is determined by the polarization direction of the UV light, and the size of the pretilt angle is dependent upon the amount of UV absorption energy.
The size of the pretilt angle is very important to the capacity of the liquid crystal display device. In the case of a large pretilt angle, the liquid crystal molecules aligned along the pretilt direction have a high response speed so that a quick dynamic picture image can be achieved. In the case of a small pretilt angle, the dynamic picture image is slowly turned and then the quality of picture image is deteriorated. In addition, since the liquid crystal molecules sensitively respond to the low driving voltage in the case of the small pretilt angle, the consumption of energy decreases.
As described above, since the pretilt angle of the polysiloxane based materials or PVCV-F is limited in 3.degree.-5.degree. larger than that of Hashimoto's method, it is impossible to obtain a pretilt angle having desired size. Further, because the pretilt angle varies abruptly according to the absorption energy, the pretilt angle cannot be precisely controlled.